1. Field of the Invention
The present invention relates to methods and apparatus for providing telephone line interfaces.
2. Background Art
There are an increasing number of devices that benefit from or require connection to a telephone line. Generally, compliance with governmental regulations is required for connection of equipment to a telephone line. The governmental regulations are intended to provide standards to ensure compatibility and to prevent harm to the public switched telephone network.
Since governmental regulations are usually issued at a national level, they usually relate to a particular country. Some countries may have regulations which may be different from, and possibly contradictory to, the regulations of other countries. Thus, a telephone line interface that meets the governmental regulations in one country may not meet the regulations of another country.
In the past, telephone equipment has typically been made to provide compatibility with only one or a limited number of countries or has been provided with a generic interface coupled to a detachable line interface module designed specifically for the particular country or countries in which it was to be used. The disadvantage to this type of telephone equipment is that it requires either complete replacement or the purchase of additional line interface modules for use in countries that have different regulatory requirements.
FIG. 9 is a diagram of a telephone line interface circuit of the prior art. Telephone line 905, which comprises tip and ring conductors, is coupled to protection devices and DC holding circuit 901. Protection devices and DC holding circuit 901 is coupled to node 908, which is coupled to a first terminal of a first winding of transformer 903. Protection devices and DC holding circuit 901 is also coupled to node 909, which is coupled to a second terminal of the first winding of transformer 903. A first terminal of the second winding of transformer 903 is coupled to node 910, which is coupled to transmit and receive circuits 902. A second terminal of the second winding of transformer 903 is coupled to node 911, which is coupled to transmit and receive circuits 902. Transmit signal TX is applied to node 906, which is coupled to transmit and receive circuits 902. Transmit signal TX is applied to node 906, which is coupled to transmit and receive circuits 902. Transmit and receive circuits 902 are coupled to node 907, which provides receive signal RX.
The first winding and the second winding of transformer 903 are isolated from each other so as to maintain high voltage barrier 904. Thus, great voltage differences between the circuits on the two sides of high voltage barrier 904 may exist without harm or interference to circuits on the other side of high voltage barrier 904.
Any signals desired to be transmitted over telephone line 905 are applied as signal TX at node 906. The signal to be transmitted is processed by transmit and receive circuits 902 and applied across node 910 and 911. The signals are coupled across coupling transformer 903 to nodes 908 and 909. The signals pass through protection devices and DC holding circuit 901 to be applied to telephone line 905. Signals on telephone line 905 pass through protection devices and DC holding circuit 101, through nodes 908 and 909 to transformer 903. Transformer 903 couples the received signals to nodes 910 and 911, which couple to receive signals to transmit and receive circuits 902. Transmit and receive circuits 902 output the receive signals at 907, which provides receive signal RX.
The windings of transformer 903 and the components associated therewith each have a characteristic impedance value. The characteristic impedance values of the components and the manner in which they are connected affects the impedance value presented to telephone line 905. For proper communication of signals, the impedances must be properly matched. Along with protection devices and DC holding circuit 901, the winding of transformer 903 coupled to nodes 908 and 909 may affect the impedance across telephone line tip and ring conductors 905. Since the impedance across the telephone line is usually subject to governmental regulations, the governmental regulations typically dictate characteristics of protection devices and DC holding circuit 901 and transformer 903. To conform with different governmental regulations, it may be necessary to replace or modify protection devices and DC holding circuit 901 and transformer 903. Such replacement or modification may be expensive and may even be impossible if the proper parts, tools, or knowledge is lacking.